This invention relates to sheet feeding apparatus and, more particularly, to a pick control arrangement for sheet feeders adapted to separate and feed individual sheets of media from a stack, especially but not exclusively plastic sheets which have a low coefficient of friction and a tendency to adhere to each other by electrostatic attraction.
In a commonly assigned, co-pending U.S. patent application entitled "SHEET FEEDING METHOD AND APPARATUS", Ser. No. 324,036, filed Mar. 16, 1989, a method and apparatus is disclosed by which individual sheets of plastic media are separated and fed from the top of a supply stack by engaging and flexing the rear end portion of the uppermost sheet in the stack to an elevated position so that a translating lifting roller may pass under the sheet, progressively separate it from the stack and move into a pressure nip relationship with a rotatable driving roller to feed the separated top sheet from the stack. The initial separation of the rear edge of the top sheet is effected by a pick member supported by a downwardly and rearwardly oriented arm pivotally connected to the pick member at one end and at the other end to a translating carriage elevated above the media stack and the translating path of the lifting roller. The pick member is shaped as an elongated body with a bottom surface adapted to lie on the rear end portion of the media stack and having a forwardly directed claw depending from the bottom surface by approximately the thickness of one sheet of the media to be fed.
Experimentation with the apparatus of the aforementioned co-pending patent application has demonstrated a high degree of reliability and effectiveness in feeding sheets of plastic media from the top of a stack of such sheets. On the other hand, the same experimentation has demonstrated a need for improvement particularly in control over movement of the pick member after one sheet is fed and the pick is returned to the top rear edge of the next sheet. For example, after the top sheet is lifted by the pick member, the translating lifting roller begins to take control of the sheet and ultimately feeds the sheet away from the pick member. The pick member then drops down against the top of the stack and is slid back to begin the next feed cycle. Because of the relatively fragile character of the sheet media surfaces, the impact of the pick member fall and sliding movement thereof on the top surface of the top sheet can and has resulted in scratching the media. Also in the event that the pick fails to engage and lift the rear edge of the top sheet of the media, necessitating a return of the pick to the rear end of the stack, even more severe damage may be inflicted by the fore and aft sliding movement of the pick on the top surface of the media.
Other problems encountered as a result of the freely pivoted pick member of the apparatus described is the obstruction represented by the pick to the insertion of a cassette containing the stack of sheet media into a position from which the media can be fed. Because in operation, the pick is required to move freely, the need for lifting it out of the way for cassette loading was evident but not easily satisfied without complicated and expensive additional mechanical components. Further, and because of the potential for failure of the pick to engage and lift a sheet as well as for the pick to lift more than one sheet, the need surfaced for a reliable detection system by which the absence of pick engagement or the feeding of more than one sheet could be detected and the detection used to return the pick for another feed cycle without damage to the media.
It is apparent, therefore, that while the sheet feeding apparatus of the aforementioned co-pending application represents a significant and an important advance in the handling and feeding of plastic sheet media, there is need for improvement.